1. Field of the Invention
This invention relates to improved techniques for detecting oxydonor generation in semiconductor wafers.
2. Prior Art
Oxydonor generation in semiconductor wafers is a well-known problem encountered in manufacturing semiconductor chips. Typically, a silicon substrate, having a silicon crystal lattice structure, is used as a P-type substrate in forming a foundation for a semiconductor device. An acceptor atom, such as boron, is used as a dopant to form a P-type substrate. Oxygen atoms, which are normally introduced interstitially within the lattice structure, tend to exchange places with silicon atoms and effectively perform as a donor dopant (therefore the term oxydonor generation). This oxydonor generation increases oxygen concentration introduced in the silicon substrate during crystal growth. Oxydonors are generated at low process temperature ranges of 350-500 degrees C., an optimal temperature being 450 degrees C.
The presence of negatively charged oxydonor atoms tend to cancel the positive charge of the acceptor dopant. When oxydonor generation occurs at a significantly high concentration, a significant change in the substrate resistivity will occur so that a part of the substrate is inverted into a N-type, resulting in a formation of a P-N junction. Such a drastic change in the substrate resistivity will cause a detrimental effect on device performance and reliability or device failures. Therefore, prior to or during the manufacturing process, oxydonor generation must be detected to prevent fabrication and subsequent distribution of defective semiconductor devices.
The oxydonor generation problem increases substantially when high resistivity substrate (those having less P-type dopant) is used to improve device performance, as is the case today with products such as EPROMS, ROMS, microprocessors and microcontrollers. Low temperature process technologies, which are increasingly used to shrink device dimensions, is prone to thermal oxydonor generation.
Various techniques exist to determine oxydonor generation in P-type substrates, such as punch-through voltage, determining forward voltage drop of P-N diodes, and sheet resistivity measurements. The sheet resistivity technique is the most reliable and accurate, however, the resistivity technique is very time consuming, usually requiring several hours. Further, special tools which are costly are required to perform the sheet resistivity measurement. The punch-through technique, although being accurate, can not be used if the N-inversion layer is formed deep in the silicon substrate as is often the case. Similarly, with the diode voltage drop technique, it becomes difficult to interpret the results if a weak inversion occurs deep in the substrate.
It is appreciated that a new technique which is highly reliable and accurate requiring no special tools and which is non-destructive to the material is needed. Further, such technique must be capable of obtaining results in a short span of time, not hours, and present no ambiguity in the interpretation of the results.